A 15-cm radius mass spectrometer which simultaneously collects positive and negative ions

International Journal of Mass Spectrometry

anti Ion Physics

41

Ekevier PublishingCompany. Amsterdam - Printedin the Ne:herlands

A 15-cm RADTUS MASS SPECTROMETER WHICH SIMULTANEOUSLY COLLECTS POSITIVE AND NEGATIVE

HARRY

J. SVEC A;\iD GERALD

IONS

D. FLESCH

institute for Atomic Research axd Department 50010

of Chemistry,

Iowa State Unirersity,

Ames,

Iowa

(Received February 6th, 1968)

A mass spectrometer has been built which simultaneously extracts positive and negative ions from a single electron beam. The instrument permits convenient, simultaneous display of positive and negative ion%pectra and has provision for direct plotting of ionization efficiency cures. This instrument will be used to study the positive-negative ion spectra of halide and oxyhalide compounds of chromium, vanadium, titanium, xenon and silicon in an attempt to obtain thermochemical information. A major problem in previous studies of these materials’ has been the characterization of the chemical processes taking place in the ion source. The strongly electronegative atoms in these molecules make the possible roIe of negative ions particularly important. Negative ions may be formed as the result of electron capture, dissociative electron capture, or ion-pair production. In addition, the reactive nature of many of these substances makes it difficult to properly assess the ionizing eIectrons’ energy because of the variable character of “contact” potentials in the ion source. This difficulty becomes a grave problem in conventional mass spectrometers when these materials are first examined for positive ions and then for negative ions by reversing appropriate voltage poiarities. A better approach is to simultaneously detect both positive and negative ions. The results with a test model consisting of two 180’ mass analyzers arranged back-to-back about a single electron gun have been reported previousIy2. A fullscale instrument has been tested during the past two years and this report describes its operation and construction features.

* Work was performed in the Ames Laboratory of tbe US. Contribution No. 2263. 1. Mass Specfromefry

Atomic Energy Commission.

and Ion Physics,

1 (1968) 41-52

TO DIFFUSION PUMP

w

I

G

,_

z

70 ION PUIAP

10 GAS LEAK

,--20 CM

a--

IO

TO ION PUMP

Fig, I. P~dtive-ncgalive ion mnss spcctrornclcr, I, collector flnngc; 2, iorr box IlitlIge; 3, collimation hnyc; 4, collector cyllnrlcr; 5, collector assembly; 6, collimation cylinder; 7, ion collitnitlion ~~sscmbly; 8, ion box-ion cxtrrlction assembly; 9, ion box-loll cxtraclion assembly mount; 10, Range pressure rings.

. _

_-

SIMULTANEOUS GENERAL

COLLECTION

OF POSITIVE

AND

NEGATIVE

ICNS

43

DESCRIPTION

The instrument was built with several moderate objectives in mind. 1. Adequate space is provided around the ion source and cohectors to provide ready accessibility and space for future modifications. 2. Both analyzers are electrically instrIated such that operation at up to 5000 V above or below ground is permissible. 3. Similarly the ion source is capable of operation at up to 3000 V above or below ground. 4. Alignment of the ion optic system is conveniently attained and easily tested. f_ The mass range l-300 is scanned magnetic&y with a single ion accelerating voltage setting and unit resolution (10 o/0valley) is achieved at mass 200. These moderate specifications were met with the arrangement shown schematically in Fig. 1, which is a top view of the instrument. The instrument consists of two 15-cm radius, 60” sectors in a horizontal plane. While the instrument is completely symmetrical and can be operated in several different modes, normally the ion source is at ground potential and the mass analyzers are at f2000 V. The two ana1yzer-s are insulated from each other (4000 V) by a IO-cm diameter, borosilicate, glass pipe tee3 which is the ion-source vacuum housing. The whole system is sup;orted at the collimation and collector flanges by mounts which allow for both vertical and horizontal adjustments. Pro\*isions are also made to allow for changes in mass-tube dimensions due to thermal expansion or contraction. Kelp bushings insulate the flanges from the mounts. The ioc-source housing is evacuated with a 40 liter set-’ mercury diffusion pump and the analyzer and collector housings are differentially pumped with 1I liter. set- r ion pumps. The ion pumps operate at ground potential and are insulated from the analyzers by 2.5-cm lengths of glass tubing sealed to Kovar’ pipes. Both magnets are also operated at ground potertiai and their pole pieces are insulated from the analyzer tube by sheets of O-75 mm Teflonj. The magnets and anaIyzer tube supports are mounted on two granite surface plates, one for each side of the instrument. These surface-plates provide excellent references for alignment procedures and also allow use of air cushions for mo-ement of the analyzers and magnets during alignment or bakeout. The granite plates are set on adjustable screws so that their surfaces can be made coplanar. The table on which the surface plates are mounted is made of aluminum and constructed in such a way that half of the table top can he displaced sideways. This makes it possible to insert or remove the glass pipe tee. A V-track in which a linear rack of ball bearings roll makes movement easy and assures precise repositioning. The entire table top area is surrounded by an aluminum grill which protects personnel from the high voltages present and provides visual inspection of the main components of the mass spectrometer. The inlet system employs dual viscous leaks with adjustable orifices and is J.

Mass

Spectrornerry

and Zon Physics, 1 (1968)

41-52

H. J. SVEC,

44

G. I). FLESCH

evacustecl by ancrther 40 liter xc-’ mercury diffusion pump. The system is of allmetal construction, except for the glass liquid-nitrogen cold trap, and is arranged so that samples can be prepared in situ and used without transfer.

ELECTRON

AND

ION SOURCES

The electron gun is a pentode. It consists of a Lament, grid, ion box, suppressor and anode and is assembied as illustrated in Fig. 2. This design was chosen because it is versatile and affords good control of the electron beam. The filament

Fig. 2. Positive-negative ion mass spectrometer electron gun. 1, ion box, 21 mm Iong; 2, metal spacer, 0.6 mm thick; 3, glass spacer, 5 mm thick; 4, suppressor, 4.5 mm hole; 5, grid, 1.2 mm hole; 6, shield, 3 mm hole; 7, anode holder; 6. filament holder; 9, anode assembly; 10, filament assembly.

is a 0.225-mm diameter tungsten wire and requires a 4.5-6.0 A current for useful emission_ The electron accelerating voltage (EAV) is variable from 0 to 85 V. The grid is a 0.25-mm thick Chrome sheet in which there is a 1.2 mm hole. Its potential is variable from - 10 to f 10 V and can be referenced to either the filament or the ion box. UsuZ: operation is t 3 to f4 V with respect to the Nament. The grid not only provides control of the electric fieid near the f&men& but it also physically shiehis the outer surface of the ion box from the electron beam. Thus, most electrons which strike the ion box must pass through the ionization region. The ion box is machined from a 6.2%mm thick stainless steel block and has a 2-mm entrance hole and a 3.Emm e_xithole for the electrons. The suppressor is an electrode similar in construction to the grid except it has a 4.5-mm hole for passage of the electron beam_ Its potential is variable from -90 to +90 V with respect to the anode. Provisions are available to monitor secondary electrons ejected from the anode. The anode is a tungsten ribbon (0.75 mm x 0.025 nun) which is heated by means of a 3.3 A cm-rentto maintain the ion source in a “baked-out” condition at all times. The anode potential is variabIe from 0 to t75 V with respect to the ion box. The hlament and anode leads pass through Armco ircn mounting blocks which not only serve to hold them firmly in place but provide a more intense and 3. Mtss Spectrum&y and Ion Physicr, 1 (1968) 41-52

SIMULTANEOUS

COLLECTION

OF POSITIVE

AND

-dEGATIVE

IONS

45

Fig. 3. Positive-negative ion mass spectrometerion source. 1, ion box, 6.25 mm thick; 2, ion box, e.xitslit, 13 mm x 0.5 mm hole; 3, lava spacer, 1 mm thick: 4, draw-out electrodes, 1.75 mm separation; 5,lata spacer, 3.5 mm thick; 6, focus electrode, 13 mm x 3.5 mm hole; 7, object slit, 7 mm x0.25 mm hole: 8. beam center electrode, 3.5 martseparation; 9, lava spacer, 9 mm thick; 10, final e.xitslit, 13 mm x0.5 mm.

uniform magnetic field for collimating the electron beam. These blocks also make positioning of the electron beam collimation magnet less critical. The back-to back arrangement of the positive-negative ion source is shown in Fig. 3. The OS-mm ion box exit slits allow a pressure ratio of S-10 between the ion box and the slit system. These ion box slits also reduce the draw-out potential field within the ioniz;ltion region to useful values. The object slit is 0.25 mm wide, and in conjunction with the O&mm collector slit, provides unit resolution at mass 200. The final exit slit of the ion source is 0.5 mm wide and limits the ion beam’s half-angle divergence, a, to l-6’_ The final exit slit is the only opening between the ion source and analyzer regions. Because of this, excellent differential pumping of the analyzer and collector regions is attained by the ion pumps.

ALIGNMEXT

The initial step in the alignment of the ion optical system is to adjust the two analyzer tubes so that they are horizontal, coplanar, and their source end axes cohnear This is relatively easy because the horizontal, coplanar surface plates on which they are mounted are the references for ah alignment measurements. Reference to Fig. 1 shows that the electrodes resporrible for the generation, collimation and collection of ions are in five assemblies. The electrodes within each assembly a-e first aligned with each other so that the system alignment involves assemblies rather than individual electrodes. Grouping of the electrodes depends on similar location, as in the case of the collector electrodes, and similar potentials, as in the case of the ion source electrodes. The problems associated with aligning the groups making up the ion source can be appreciated by referring to Fig. 4. The collimation assemblies are held in J. Mass Spectrometry

and Ion Physics, 1 (1968) 41-52

H. J. SVEC,

46

G. D_ FLESCH

Fig. 4.. Positive-negath e ion mass spectrometer ion source region. 1, z axis adjustment; 2, x axis adjustment; 3, v axis adjustment; 4. ion box insulator; 5, bellows; 6, collimation cylinder; 7, ion collimation assembly; 8, ion box-ion cxtrxtion assembly.

place on positioning pins on the collimation cylinders by means of lockable screw metal clips. Similar clips also hold the cylinders in place on the collimation flanges after they have been rotated to make the slits of the assemblies vertical. A similar arrangement and alignment procedure are used for the collector assemblies. Thus, once the system is aligned, these four assemblies can be removed and replaced with confidence that the alignment will be preserved_ W&ring the ion box-ion extraction assembly with the collimation assemblies is more difficult_ As Fig. 4 indicates, the glass tee is a secondary reference for p!acement of the ion box. This requires that the flange surfaces of the glass tee be ground accurately and that these ground surfaces be perpendicular to the granite surface plates when the respective flanges are tightened_ The short metal bellows allows for slight non-parallelism of the two collimation end surfaces and also small dimensional variations in glass tees. The mount to which the ion box-ion extraction assembly is attached is capable of movement in three dimensions, each of which is provided with a reference scale. This movement plus use of an alignment jig and visual checking make it possible to accurately align the ion box-ion extracticn assembly with the rest of the mass spectrometer.

EiEClRICAL Ml

AEiD EJXCl-RONIC FEATURES

electrical f&-throughs

1. Mass Spectrometry

ad

into the mass tubes are demauntable. Two scven-

Ion P/z>sics, I (1968) 41-52

SIMULTANEOUS

COLLECTION

OF POSITIVE

AND

NEGATIVE

IONS

47

lead feed-throughs previously described’ are used at the ion box Sanges. The feedthroughs at the two collimation flanges and two zollector flanges are all big!- impedance, single leads. Fig. 5 shows the construction details. A BNC6 connector is attached to the tightening nut to allow quick electrical connections. Four of these feed-throughs are provided at each of the ccllimation flanges and six are provided at each of the collector flanges. Their use at the collimation flanges makes it possible to monitor ion currents at the ion source collimating electrodes_

Fig_ 5. Low-leakage feed-through 3, slip ring, brass; 4, f&-through, Iess steel.

assembly. 1, BNC connector, modified; 2, tightening nut, brass; ko\ar and stainless steel: 5, gasket, aluminum; 6, flange, stain-

The various ion-source electrode potentials are supplied from voltage dividers across the high-voltage supplies_ The potential at each electrode is continuously and independently variable from ground to 2500 V. This allows complete freedom for selecting ion-source operating potentials. Commercial electronic circuits are used where possible. These are: two Ultek’ model 60-062L ion pump power supplies, one Hughes8 IGC-IO1 ionization gauge control, two Hastings9 model CVT-6 compact vacuum controllers, two NJE” model S-327 high-voltage supplies, two Keithle#’ model 6OOA electrometers, two Keithley” model 601 electrometers, one Leeds and Northrup” Speedomax strip chart recorder, and one Houston’ 3 model HR 96, X-Y recorder. The two 15-cm radius, 60’, 1 I-mm pole gap electromagnets (Nuclide Corp.‘” model EM66@-4) are powered by loc&y built, regulated current supplies (250 .mA, 2200 Q load, 10 p-p-m_ regulation) and can produce fields up to 8000 gauss. The magnet current is adjusted by means of a forty-turn helical control potentiometer. The potentiometer is driven by a Heller” model 2T60 variable speed motor equipped with manual over-ride. Two auxiliary, ten-turn helical control potentiometers are available and may be alternately switched into the control circuit to provide rapid scanning between three differ tit m/e ions. Fig. 6 is a block diagram of the magnet current supply. The emission regulator was also locall: designed and built. It uses solid-state circuit components and the filament is heated by direct currant. Various switching

9. J. SVEC,

G. D. FLESCH

Fig. 15.BIock diagram of the magnet current sup~;.y. GRID FfL..WENT

I

ION

90x

i

SUPFRESSOR

i

I

I r

I’ !

I I

I

I

I Ii-iFig. 7. Block d&an

lL

CHOIIE-PLUS-ION-W

CURhSNTS

CONSTANT

of the emission regulator.

options allow for operation of the electron gun in several modes. Variable potentiaIs are available to each of the electrodes in the assembly and all currents to each electrode are monitored. A block dia_gra&of the components of the emission regylator is shown in Fig. 7. The regulator maintains a constant voltage drop acrozs sensing resistors RA (ion box current plus anode current constant), I&

SIMULTANEOUS

(anode

COLLECTION

current constant),

OF POSlTIVE

or R,

@ament

AND

potentiometer.

ranges, O-2,0-20,

and &200

0.01 % from O-10 -4. Battery-operated

49 The electron

by suitable meters or by means

Electron currents are controlled @.

IONS

heater c*urrent constant).

currents to each of the electrodes can bc monitored of a recording

NEGATIVE

to 0.05 ok in three

and the filament heater current is controlled

dectrometer

amplifiers

are used as ion detectors

to

because

they can operate at analyzer tube potentials without requiring power line isolation. In order to safely record ion currents on a strip chart or X-Y

recorder at ground

potential a dc-dc converter was developed (details to be published). This electronic device accepts the CL1 V dc electrometer output voltage at the ion collector potential

(up to f3000

V) and provides a corresponding

O-l V dc output voltage

at ground potential. The converter has a 0.25-set time constant and over the O-I V range deviation of the output voltage from linearity is less than 0.5 mV. A block diagram of its components is shown in Fig. 8.

Fig. 8. Block diagram

of the dc-dc converter.

OPERATION

To obtain the spectra of positive and negative ions simultaneously, the emission regulator is usually operated in the constant anode-plus-ion box current mode with 20 @ of ‘70-V electrons. A grid potential of f4 V with respect to the filament is used. The suppressor is at ion box potential and the anode is + 75 V with respect to the ion box. The ion-source electrode potentials for positive ions are: ion box, ground; draw-out, -300 V; focus, -400 V; object slit, -2000 V; beam center, -2000

V; analyzer tube, -20%)

V. Similar potentials,

but of opposite

polarity,

are used for the negative ion SOW electrodes. A magnetic scan from m/e 12-120 on either side requires about 8 min. The scan speed is limited by the response time of the present ion detection system. A positive-negative spectrum of chromyl fluoride, CrOzFt,

is shown in Fig. 9. J. Mass

Specfrometry

and ion Physics,

1

(1968) 41-52

Z-X.J. SVEC,

50 1

1

I

I

I

i

I

G_ D.

FLESCH

1

Fib 9. Poritive-negative ion mass spectrum of chromyl fluoride. Pressure: 1.3 x IO+ torr; ionizink current: 40 PA; electron energy: 70 eV_

The electron gun and ion source electrode potentials used for ionization efkiency (E) experiments are the same as those for mass spectral operation except that the electron currents are -C10 a. Ionization efficiency data are automatically recorded by means of an X-Y recorder_ The EAV is controlled by a motorELECTRON

ENERGY

W’.XTSl

Fig- 10. Ionization efficiency curves illustrate the use of a bucking voltage and variable sensitivity to pIot various electron energy spans. Gas: CrO,F,; electron current: 2 @. J- Mass Specrrome~

and Ion Phxics,

1 (1968) 41-52

SIMULTANEOUS

COLLECTION

OF POSITIVE

AND

NEGATIVE

IONS

fi

driven, fifteen-turn helical. potentiometer_ An accurate 1.0090 y0 voltage divider makes it possible to measure accurately the EAV with a Rubicon’” model 2710 potentiometer. This measure of the EAY may be displayed simuhaneously on the X-Y recorder_ Use of a bucking voltage with the recorder and potentiometer makes it possible to set any nominal electron energy as the “zero” for the recorder display. The variable sensitivity of the X-Y recorder allows the choice of another higher nominal energy as the full-scale recorder deflection. Thus, for precise inspection of IE data any portion of an IE curve can be expanded to full-scale recorder display. An example of this is shown in Fig. 10 where the IE curves for CrCF+and CrO, Ftf from chromyl fluoride are shown for 6.25 V spans near the energ& of interest. The minimum possible span is 1.25 V. Electron currents up to 5 JLA can be effectively controlled bjr the emission regulator from 85 to 2 eV nomir~ energy. For tower-energy electrons, constant flament heater current is applied and electron currents of 0.25 fi arc available to a nominal energy of 0 eV.

RESULTS

Preliminary studies of chromyl fluoride, vanadium oxytrifluoride, and sulfur hexafiuoride have shown that the instrument simultaneously extracts positive and negative ions from a single electron beam. Unit resolution (10 0/0valley) is achieved at m/e 200 and useful IE data can be obtained. The positive-negative mass spectrum for chromyl fluoride is shown in Fig. 9. The presence of negative ions using 70 V electrons at first appears to be evidence for ion-pair production. However, preliminary studies indicate that these ions are due to capture of slow electrons_ These slow electrons are either electrons ejected during the production of positive ions and/or secondary electrons resulting from bombardment of the electron gun structures by the ionizing electron beam. The occurrer,ce of true ion-pairs appears to be relatively rare for these compounds. Little interaction of the two sides of the ion source is observed. For example, a variation of the high-voltage supply of one analyzer from 2000 to loo0 V affects the intensity of ion currents observed in the other analyzer by less than 2%. Generally only minor refocussing adjustments are necessary when such Large voltage changes arc made. Ionization effi&ncy expe.+ents indicate that the instrument operates weli for Positive and nqnative ions_ It has been observed that the electron energy at which dissociative capture takes place in chromyl fiuoride to form CrO,Fis 2.44 eV when SF,- (resonance captzlre energy 0.07 eVr’) is used as the erectron energy reference, and 2.45 eV when A+ is the energy reference_ Because the actual value of the SF, resonance capture energy may lie between 0 and 0.1 eV1’, one concludes that the maximum variation of electric potential within the ionization region of I_ Mass Spectrontctry

cmndIon Physics, 1 j196S) 41-52

52

H. J. SVEC,

G. D. FLESCH

the ion scurce is 0.1 V. A secondary effect of this small variation of electric potential across the ionization region is that positive-ion trapping can be observed when ionizing currents as low as 30 j~.4 are employed.

SmfARY

A mass spectrometer which simultaneously extracts positive and negative ions from a single electron beam is described. Its features include: ready accessibility to the ion source and collectors, convenient attainment of alignment, simultaneous display of positive and negative ion spectra, and direct plotting of ionization efficiency data.

REFERENCES I

2 3 4 5 6 7 8 9 :O 11 12 13 I4 15 16 17

H_ J_ SVEC AND G. D_ FLESCX,Science, 132 (1963) 954; G. D. FUSCH AWD H. J_ SVEC,J_ AmCfzem.SOL, 81 (19593 1787. rJ. D. FLESCH AMY H. 1. SYEC,Rec. ScL Insrr., 34 (1963) 897. Fisher and Porter Co., 1205 South Eighth Ave., hiaywood, III. 60153. CadilIac Plastic and Chemical Co., 151 I1 Second Avenue, Detroit, Mich. 48203. The Carborundum CO., Electronics Division, P-O_ Box 311, Latrobe. Pa. 15650. Aliied EIectronics Carp_, 100 N-Western Avenue, Chicago. 111.60680. Ultek Division, Perk&Elmer Corp., 1212 Washington Ave., Wilmette, 111.60091. Hughes Airrraft Co., Vacuum Tube. Products Dhision, 2020 Short Strec+. Oceanside, Calif92057. Bastings-Raydist, Inc.. Newcomb Avenue, Hampton, Vz. 23361. NJE Corporation, 20 Boright Ax, KcniIworth, N. J. 07033. Keithley InstrumentsInc., 12415 Euclid Ave_, Cleveland, Ohio 44106. Leeds.and Northrup Co.. 4970 Stenton Ave., Philadelphiz, Pa. 19144. Houston Omnigraphic Corp., 4950 Terminal Drike, Bellaire, Texas 77401. Nuelide Corp.. 642 East College Ave, State College. Pa_ 16801_ Gerald Hcller Co., 2673 S. Western Strezt, Las Vegas, Nevada 89102. HonqweU, Test instruments Div., 4800 E. Dry Creek Road, Denver, Cola. 80217. W- M. Hrcm oh?) R. E FOX, 1_ Chem. Whys., 25 (1956) 642.

J. Mass Spectrumdry and ZonPhysics, 1 (1968) 41-52